Why are the straight lines in my model distorted when UV unwrapped?
Maybe all the planes of the fireplace are not at right angle to each other. You can confirm with this script (select the fire place before executing it; the info will be printed on console).

All the angles should be close to 90 (or 0). 

import bmesh
import bpy
import math

obj = bpy.context.object 
me = obj.data

if me.is_editmode:
    bm = bmesh.from_edit_mesh(me)
else:
    bm = bmesh.new()
    bm.from_mesh(me)

for f1 in bm.faces:
    angleInfo = (f1.index, [round(math.degrees(f1.normal.angle(f2.normal)), 3) \
        for e in f1.edges for f2 in e.link_faces if f1 is not f2])
    print(angleInfo)